Known processing methods of oxidative hydrometallurgy are commonly used in many different applications. These applications generally require oxidation conditions of high temperature and pressure, and require substantial supplies of oxygen. For example, base metals such as copper, nickel and zinc can be recovered by hydrometallurgical processes which usually embody pretreatment, oxidative leaching, solid/liquid separation, solution purification, metal precipitation or solvent extraction and electrowinning.
According to conventional technology, oxidative processes usually require severe physico-chemical conditions in order to achieve acceptable rates of oxidation and/or final recoveries of metal. Under these severe physico-chemical conditions, which often mean temperatures in excess of 200.degree. C. and total pressures in excess of 2000 kPa, the chemical reactions which occur use large quantities of oxygen, both on stoichiometric considerations and in practice where amounts in excess of stoichiometric requirements are often used.
The typical oxidative hydrometallurgical processing methods referred to above generally have oxidation reactions that are carried out in multicompartment autoclaves fitted with agitators. In order to withstand the generally highly aggressive conditions of the reactions, the autoclaves are very costly to install and maintain. These vessels must be capable of withstanding high pressure, and linings of heat and acid resistant bricks often need to be used. The agitators are generally made of titanium metal or other, more costly alloys, and the pressure relief systems utilised are also costly and require high maintenance. These high costs, together with the sophistication of the technology (skilled operators are generally required), detract from the wider acceptance of high pressure/high temperature oxidation, particularly for use in remote areas or by small to medium size operators.
U.S. Pat. No. 5,232,491 (assigned to Dominion Mining Limited) describes a method of activating a mineral species in order to alleviate the difficulties and expenses referred to above with the traditional processing methods of oxidative hydrometallurgy, and in particular with the oxidative leaching of a mineral species. In the method of U.S. Pat. No. 5,232,491 the mineral species is activated by fine or ultra fine milling prior to processing by methods of oxidative hydrometallurgy. The milled mineral species may be subjected to oxidative leaching under relatively mild conditions of pressure and temperature due to the milling producing minerals which are activated, and which thus react far more readily with oxidants such as oxygen. Furthermore, the oxidative leaching is able to be conducted under conditions using less oxidant than that required for complete sulphur oxidation to sulphate.
While the method as described in U.S. Pat. No. 4,232,491 is applicable to any mineral species, such as sulphide minerals, arsenide minerals, telluride minerals, or mixed minerals of sulphides, arsenides or tellurides, the method is particularly useful for the activation and subsequent leaching of sulphide minerals.
However, copper sulphide minerals, and in particular chalcopyrite, have been difficult to treat by oxidative hydrometallurgy in sulphate systems. Indeed, even the method described by U.S. Pat. No. 5,232,491 has had limited success when applied to copper sulphide concentrates containing chalcopyrite.
In this respect, when practising the method of U.S. Pat. No. 5,232,491 on chalcopyrite it has been found that the dissolution of the chalcopyrite is often incomplete. Although the precise reason for this has not been determined with certainty, it is believed that very fine coatings build up on the surface of the chalcopyrite (during leaching), thus preventing the relevant reactions going to completion. This results in long reaction times and usually low recoveries.
Thus, processing options for the treatment of chalcopyrite-containing concentrates have remained somewhat limited. Such options include the normal pyrometallurgical option, namely smelting, followed by a hydrometallurgical refining process, or alternatively the solely hydrometallurgical route which requires leaching with a highly concentrated chloride-based aqueous media. This latter type of system has not proven to be economically successful due to problems with the materials of construction (caused by the highly corrosive aqueous media), and their inability to recover a commercial product which does not require further refining prior to its final downstream processing.
Indeed, such chloride-based leaches rely on high concentrations of chloride ions, usually greater than 1M (or 35 g/L), and more typically 5 to 10M (or 175 to 350 g/L). The copper dissolved in such chloride-based leaches is therefore present as the chloride.
U.S. Pat. No. 4,971,662 is an example of a combined conventional grind and chloride-based leach system where chloride ion concentration is less than 75 g/L, and conditions are maintained so as to extract cupric copper in a dominantly chloride environment which can then be transferred to a sulphate solution using conventional solvent extraction techniques.